Test facility design for the measurement and modelling of candu reactor moderator flow and temperature distributions

In CANDU reactors the heavy water moderator may serve as a temporary heat sink over a long period of time under certain beyond design basis accidents. Under normal conditions, the moderator water inside the calandria experiences volumetric heating from neutron and gamma radiation and this heat is removed via the moderator cooling system. Since the local heat loads may be large, there are strong buoyant forces in combination with the momentum forces at the moderator inlets. CANDU moderator flow and temperature distribution predictions form an important part of the pressure-tube / calandria-tube integrity calculations during some postulated design basis accidents. The capability of the moderator to sustain heat removal from the fuel channels is dependent on the initial conditions of the moderator flows and temperatures prior to the accident. Hence there is need to accurately predict the normal operating conditions inside the large calandria vessel as well as the transient behavior during the postulated accident. In the small-scale facility, the flow enters through inlets near the top of the vessel and exits at the bottom of the cylindrical vessel. Heat is supplied to an array of cylindrical electrical heaters placed inside the vessel in a similar fashion as the calandria tubes in a CANDU reactor. The facility is designed to allow the investigation of a number of flow / heat power input combinations to achieve different flow regimes inside the vessel: forced, buoyancy dominated and mixed convection regimes. Temperature measurements for a reference power level and flow rate combination were compared to simulations as part of the commissioning phase for the facility.